Centric jaw relation (CR) is a complex and controversial concept in dentistry. Main reason for the controversies is because its definition changed many times during last ten decades. Position of the head of the condyle during centric jaw relation position is the reason for most of the controversies. With the advancements in the field of oral radiology now it is understood that the head of the condyle does not attain most retruded unstrained position but it attains most anterior and superior position in the glenoid fossa. Centric jaw relation remains complex because the latest Glossary of Prosthodontic Terms (GPT)-8 continues to give seven definitions for centric jaw relation and some of the definitions are contradictory to each other. Another reason is as per latest GPT-8 the centric jaw relation definition, which is widely accepted, relates CR to so many clinically invisible parts making it difficult for the operator to follow its description in clinical dentistry. There is no device which will help the operator to guide the head of the condyle to attain this position as per the description given in the definition. Recording CR is very important step because it is the only relationship of the mandible to the maxilla which is repeatable, reproducible and recordable.
CR is defined as per the latest GPT-8 and widely accepted, “the maxillo-mandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective discs with the complex in the anterior-superior position against the slopes of the articular eminences. This position is independent of tooth contact. This position is clinically discernible when the mandible is directed superiorly and anteriorly. It is restricted to a purely rotary movement about the transverse horizontal axis”.
Apart from recording centric jaw relation other important relationship of the jaws with the Temperomandibular joint (TMJ) which is known as orientation jaw relation which is to be recorded using a facebow. Maxilla is a part of the cranium and is a fixed entity when the teeth of both jaws come in contact; mandible gets related to the maxilla so that the entire craniomaxillary complex is articulated with a moving bone, which is the mandible. The maxilla is oriented uniquely to the TMJ/cranium and this positioning differs from individual to individual. The anatomy of maxilla and the temporomandibular joint varies from person to person. Recording of the opening axis of the patient and its transfer to the articulator is very important for the occlusion developed on the articulator to remain same when the prosthesis is transferred to the patient's mouth, is the logic behind recording orientation jaw relation.
Centric jaw relation is an old concept in dentistry. There are over 26 different definitions of centric jaw relation since 1926. It is a position which is the only position of the mandible that is repeatable, recordable and reproducible. According to old definition of the centric jaw relation, the location of both condyles was considered to be attaining the most retruded position, however now it is understood that it attains most anterior and superior position in the glenoid fossae. The accuracy of recording is highly dependent on the individual understanding of the concept, skills, judgement, method chosen by individual dentist, their experience, etc. Centric jaw relation is the mandibular jaw position in which the head of the condyle is situated as far anterior and superior as it possibly can within the glenoid fossa.
Dentists use various conventional methods and devices to obtain precise location and recording of centric jaw relation which includes physiological methods like tactile or interocclusal check record method, pressure less method, pressure method or functional methods like needlehouse method, patterson method or graphic methods like intraoral method, extraoral method, and radiographic method. In order to record the centric jaw relation, it is important that both condyles be seated in their most superior and anterior position in the glenoid fossae. However, there exists no single device or instrument to achieve a precise location and recording of a centric jaw relation in edentulous and dentulous patients that can be accepted universally. There does exist a precise single method i.e. graphic recording which is most commonly used in edentulous patients with good alveolar ridges, however it has its own limitation and cannot be used for all the patients.
Similarly, the face bow transfer which establishes the relationship between the maxilla and the TMJ. i.e the relationship of the maxilla to the opening axis so that the maxillary cast can be mounted on the articulator in the correct anatomical position is another important aspect in dentistry and face bow is a device that is used to record the relationship of the jaws to the temporomandibular joints or the opening axis of the jaws and to orient the cast in the same relationship to the opening axis of the articulator. There are many existing face bow recording devices for achieving the required face bow transfer. However, there is no single device in prior art that allows a combination of centric jaw relation recording and face bow recording and its transfer in one sitting of the patient.
U.S. Pat. No. 3,643,332A discloses a method for measuring the movements of the lower jaw (mandible) relative to the upper jaw by selecting two points at a fixed distance apart on the horizontal axis of rotation of the subject's mandible, one on each side of subject's head and at substantially equal distances from the subject's head. This prior art document allows only face bow transfer and does not record centric jaw relation.
Another patent document EP950384B1 discloses an apparatus for tracing to record a centric jaw relation of a patient, particularly by a tracing apparatus in which a stylus is used to record a centric jaw relation of a patient for the fabrication of a prosthesis or dentures is coupled to a nut-shaped ball supported by a base plate mounted to a dentition and a support plate threadedly coupled to the base plate in such a fashion that it can be selectively pivotable, thereby eliminating use of any separate dentition supporting wires. However, the disclosed apparatus may cause injury to the patient due to the presence of the stylus and there could also be chances of uneven positioning of the apparatus that would ultimately give inaccurate recordings.
Another prior art document U.S. Pat. No. 3,750,289A discloses the centric jaw relation device that comprises of an upper frame and a lower frame which are hinged together along the condylar axis by a joint which permits relative movement only by rotation in a medial plane. However, this prior art references can only be used in case of edentulous patients and can only have its application in face bow transfer and not for centric jaw relation recording per se. Further, the devices of the prior art have a complex structure, and may cause injury to the patient. These devices though may provide satisfactory jaw relation recordings and face bow transfer, but require multiple sittings and appointments of the patients for face bow transfer and centric recording procedure. There is no device in art that provides both the procedures in one appointment. Further, most of the prior art devices for recording of centric jaw relation can only be used in case of edentulous patients. It is found that there has not been a single attempt to develop a device for recording centric jaw relation as per the latest definition as per GPT-8.
Thus, to achieve an accurate and precise centric jaw relation recording as well as face bow recording and transfer or orientation jaw relation simultaneously of subjects is an essential criterion for restoration of function, facial appearance, and maintenance of subject's oral health. Inaccurate centric jaw relationship will eventually lead to failure of a prosthesis leading to re-fabrication, which is a time consuming and a costly affair. Therefore, there exist a need for a centric jaw relation recording device that gives accurate recording of centric jaw relation and which allows orientation jaw relation as well, thereby avoiding multiple appointments for the subjects and a device that is safe and that can be used in both, dentulous as well as edentulous subjects.